1. Field of the Invention
This invention relates to text processing apparatus in which text is formatted onto a display screen by means of a two-stage formatting process.
2. Description of the Prior Art
Modern text processing apparatus comprises a keyboard by means of which an operator can create and edit text documents and a display screen on which the document being created or revised can be displayed. Provision may also be made for filing, printing or processing the document created on the screen or alternatively communicating it electronically over a communications link to be viewed, filed, processed or printed at a remote location.
The IBM 3730 Distributed Office Communication System incorporates a so-called shared logic system in which a number of display terminals are connected to a local minicomputer or controller and exemplifies such text processing apparatus. In the IBM 3730 system, documents are stored electronically in permanent storage on an external media from where they are accessed in response to operator commands via keyboard entry.
When an operator wishes to view a document, the document is placed in working storage assigned to that operator and the document, or part of it, is then transferred under control of a software editor from working storage in an IBM 3791 controller to a display refresh buffer of an IBM 3732 text display terminal. The contents of the refresh buffer are displayed directly on the screen. Text keyed into the system by the operator is entered directly into the refresh buffer and therefore appears on the screen as it is keyed. Periodically, the contents of the refresh buffer are sent to the controller and the document in working storage updated under control of the software editor. Normally, document update occurs in response to an operator `enter` command from the keyboard. When the document in working storage has been edited to the operator's satisfaction, a `store` command is used to store the document in permanent storage.
From the foregoing it is seen that there may be one version of the document permanently stored on the external media; a somewhat more up-to-date version in working store; and a fully updated version in the refresh buffer and thus on the screen.
The provision of a microprocessor and random-access memory (RAM) in a display terminal has made it generally possible to take function out of the controller and put it instead in the terminal itself. If follows that it is desirable to arrange for editing and formatting processes to be performed automatically entirely in the display terminal and at a speed so as not to interfere with keystroke entry by the operator. One proposal described in our co-pending European Application No. 80302196.3 involves arranging for a single unformatted version of the document stored to be updated at each keystroke. Formatting and mapping to the screen is then a one way process which is performed as required, for example, after each editing action. This separation of the document formatting and editing processes would eliminate the need to store several versions of the document and significantly simplify the hardware and software.
In the IBM 3730 however, documents are stored on the external media in System Network Architecture (SNA) Character Set (SCS) format whereas the version of the document in working store is in a format between SCS and that sent to the IBM 3732 to display part of the document. The manual entitled IBM 3730 Distributed Office Communication System: Application Programmer's Reference (GA33-3031-1), copyright 1980, published by International Business Machines Corporation describes the SCS data stream in some detail.
Transformation from SCS to displayed form is performed sequentially through the document from top to bottom. Transformation from working store format to 3732 format is done on parts of the document as they are displayed. Changes to the document by the operator require reverse transformation repeatedly and in fragments during editing. This could be avoided by making the keyed changes directly on the SCS form and making forward transformation to display the updated document as described in our aforesaid co-pending application. The problem with this arrangement is that the transformation must be done at a speed of the same order as keystroking and the microprocessor may not be sufficiently powerful to perform this transformation with the formatting constraints of the IBM 3730.